Objection 1. It would seem that incontinence pertains not to the soul but to the body. For sexual diversity comes not from the soul but from the body. Now sexual diversity causes diversity of incontinence: for the Philosopher says (Ethic. vii, 5) that women are not described either as continent or as incontinent. Therefore incontinence pertains not to the soul but to the body.
Objection 2. Further, that which pertains to the soul does not result from the temperament of the body. But incontinence results from the bodily temperament: for the Philosopher says (Ethic. vii, 7) that "it is especially people of a quick or choleric and atrabilious temper whose incontinence is one of unbridled desire." Therefore incontinence regards the body.
Objection 3. Further, victory concerns the victor rather than the vanquished. Now a man is said to be incontinent, because "the flesh lusteth against the spirit," and overcomes it. Therefore incontinence pertains to the flesh rather than to the soul.
On the contrary, Man differs from beast chiefly as regards the soul. Now they differ in respect of continence and incontinence, for we ascribe neither continence nor incontinence to the beasts, as the Philosopher states (Ethic. vii, 3). Therefore incontinence is chiefly on the part of the soul.
I answer that, Things are ascribed to their direct causes rather than to those which merely occasion them. Now that which is on the part of the body is merely an occasional cause of incontinence; since it is owing to a bodily disposition that vehement passions can arise in the sensitive appetite which is a power of the organic body. Yet these passions, however vehement they be, are not the sufficient cause of incontinence, but are merely the occasion thereof, since, so long as the use of reason remains, man is always able to resist his passions. If, however, the passions gain such strength as to take away the use of reason altogether--as in the case of those who become insane through the vehemence of their passions--the essential conditions of continence or incontinence cease, because such people do not retain the judgment of reason, which the continent man follows and the incontinent forsakes. From this it follows that the direct cause of incontinence is on the part of the soul, which fails to resist a passion by the reason. This happens in two ways, according to the Philosopher (Ethic. vii, 7): first, when the soul yields to the passions, before the reason has given its counsel; and this is called "unbridled incontinence" or "impetuosity": secondly, when a man does not stand to what has been counselled, through holding weakly to reason's judgment; wherefore this kind of incontinence is called "weakness." Hence it is manifest that incontinence pertains chiefly to the soul.
Reply to Objection 1. The human soul is the form of the body, and has certain powers which make use of bodily organs. The operations of these organs conduce somewhat to those operations of the soul which are accomplished without bodily instruments, namely to the acts of the intellect and of the will, in so far as the intellect receives from the senses, and the will is urged by passions of the sensitive appetite. Accordingly, since woman, as regards the body, has a weak temperament, the result is that for the most part, whatever she holds to, she holds to it weakly; although in rare cases the opposite occurs, according to Proverbs 31:10, "Who shall find a valiant woman?" And since small and weak things "are accounted as though they were not" [Aristotle, Phys. ii, 5 the Philosopher speaks of women as though they had not the firm judgment of reason, although the contrary happens in some women. Hence he states that "we do not describe women as being continent, because they are vacillating" through being unstable of reason, and "are easily led" so that they follow their passions readily.
Reply to Objection 2. It is owing to the impulse of passion that a man at once follows his passion before his reason counsels him. Now the impulse of passion may arise either from its quickness, as in bilious persons [Cf. I-II, 46, 5], or from its vehemence, as in the melancholic, who on account of their earthy temperament are most vehemently aroused. Even so, on the other hand, a man fails to stand to that which is counselled, because he holds to it in weakly fashion by reason of the softness of his temperament, as we have stated with regard to woman (ad 1). This is also the case with phlegmatic temperaments, for the same reason as in women. And these results are due to the fact that the bodily temperament is an occasional but not a sufficient cause of incontinence, as stated above.
Reply to Objection 3. In the incontinent man concupiscence of the flesh overcomes the spirit, not necessarily, but through a certain negligence of the spirit in not resisting strongly.
Source: Summa Theologica (New Advent)